The systematic change in a trait with size is a concise means of representing the diversity and organization of planktonic organisms. Using this simplifying principle, we investigated how interactions between trophic levels, resource concentration, and physiological rates structure the planktonic community. Specifically, we used 3 size-structured nutrient-phytoplankton- zooplankton models differing in their trophic interactions, ranging from herbivorous grazing on one size class to omnivorous grazing on multiple size classes. We parameterized our models based on an extensive review of the literature. The maximum phytoplankton growth, maximum microzooplankton grazing, and phytoplankton half-saturation constant were found to vary inversely with size, and the nutrient half-saturation constant scaled positively with size. We examined the emergent community structure in our models under 4 nutrient regimes: 10, 20, 25, and 30 μM total N. In all models under all nutrient conditions, the normalized biomass of both phytoplankton and microzooplankton decreased with increasing size. As nutrients were in creased, phytoplankton biomass was added to larger size classes with little change in the extant smaller size classes; for microzooplankton, spectra elongated and biomass was added to all size classes. The different grazing behaviors among models led to more subtle changes in the community structure. Overall, we found that phytoplankton are top-down controlled and microzooplankton are largely bottomup controlled. Sensitivity analyses showed that both phytoplankton and microzooplankton biomass vary strongly with the size-dependence of the maximum grazing rate. Therefore, this parameter must be known with the greatest accuracy, given its large influence on the emergent community spectra.

Estimates of growth and grazing mortality rates for different size classes and taxa of natural picophytoplankton assemblages were measured in mixed-layer experiments conducted in 3 regions of the eastern Pacific: the California Current Ecosystem, Costa Rica Dome, and equatorial Pacific. Contrary to expectation, size-dependent rates for cells between 0.45 and 4.0 μm in diameter showed no systematic trends with cell size both in and among regions. For all size classes, mean ± SD growth rates ranged from .0.70 ± 0.17 to 0.83 ± 0.13 d-1 and grazing rates between .0.07 ± 0.13 and 1.17 ± 0.10 d-1. Taxon-specific growth rates for Prochlorococcus ranged from 0.17 ± 0.12 to 0.59 ± 0.01 d -1, for Synechococcus from 0.68 ± 0.03 to 0.97 ± 0.04 d-1, for picoeukaryotes from 0.46 ± 0.13 to 1.03 ± 0.06 d-1, and for all cells combined between 0.45 ± 0.03 and 0.65 ± 0.02 d-1. For grazing, Prochlorococcus rates ranged between 0.02 ± 0.12 and 0.66 ± 0.02 d-1, Synechococcus rates between 0.24 ± 0.08 and 0.92 ± 0.05 d-1, for picoeukaryotes between 0.19 ± 0.10 and 0.78 ± 0.09 d-1, and for all cells between 0.16 ± 0.05 and 0.75 ± 0.02 d -1. When comparing rates among taxa, only Prochlorococcus had consistently lower rates than Synechococcocus in all regions. No other trends were apparent. Temperature relationships based on the Metabolic Theory of Ecology were able to explain more of the variability among grazing rates than among growth rates for each taxon considered. © 2014 Inter-Research.